This application is based on and claims priorities from Japanese Patent Application Nos. Hei. 3-349304 filed Oct. 31, 1991 and Hei. 4-110831 filed Apr. 3, 1992, the disclosures of which are incorporated by reference herein.
The present invention relates to a surface reflecting mirror having a surface reflecting multilayer film that is used in optical products such as cameras, telescopes and microscopes.
In a surface reflecting mirror used in optical products such as cameras, telescopes and microscopes, aluminum is most commonly used as a reflecting material. However, sufficient resistance to scratching, resistance to humidification, etc. cannot be obtained simply by forming an aluminum reflecting layer on a substrate. Conventionally, this problem has been solved by forming a protective layer of an oxide of silicon, magnesium fluoride, etc.
Silver, which has a high reflectivity over a visible to near infrared range, is commonly used as a reflecting material for a high-reflectivity surface reflecting mirror that is used in optical products. However, a single layer film of silver is inferior in film adhesiveness, resistance to humidification, resistance to sulfurizing, etc. In order to improve these characteristics, a multilayer film is formed by the silver single layer film, an under layer and protective layers.
The durability of this type of high-reflectivity surface reflecting mirror is evaluated by an accelerated test for resistance to humidification at 40.degree.-60.degree. C. When the above high-reflectivity surface reflecting mirror was subjected to a test for resistance to humidification at 60.degree. C. and 90% RH for 24 hours, it was sometimes the case that the laminate films peeled off from the substrate to cause point-like defects. It is considered that the point-like defects are caused by the thermal expansion or the swelling by damping of the resin.
Although the point-like defects gradually disappear if the reflecting mirror is again placed in a usual atmosphere, they will deteriorate the film adhesiveness to thereby lower the durability of the reflecting film.
In recent years, due to the development of ultra-high-precision die-machining tools and the improvements of the injection molding technique, resins have come to be used as optical members. In particular, engineering plastics such as a polycarbonate resin and a polyacetal resin are superior in durability, and can be used at a high temperature. However, even with the engineering plastics the problems originating from the occurrence of the point-like defects have not been solved yet.